This invention relates generally to weighing systems for vehicles and more particularly to an onboard weighing system for a truck-trailer combination in which the trailer utilizes a single point suspension. The invention also relates particularly to an onboard weighing system for measuring the load imposed on a fifth-wheel table or log truck bunk by the trailer.
In the trucking business profitable operation usually requires making optimal use of the vehicle and such use requires carrying as much load as possible. However, care must be exercised in loading a truck because the weight of a commercial vehicle operating on the highways must be known for safety reasons and for compliance with state and federal weight regulations. Those regulations specify the maximum total vehicle weight and maximum weight per axle for a wide variety of vehicle categories. If a vehicle is found to be in violation of those regulations the operator can be fined and can be required to reposition or remove cargo from the vehicle to bring it into compliance. Such violations can be costly and can seriously disrupt shipping schedules.
The most common device used for weighing trucks is a platform scale. Such scales are installed at official state weighing stations and are also available at privately operated service centers. Also many companies choose to install platform scales at their distribution centers for their own convenience. In order to weigh a truck the operator drives one axle at a time, or the entire vehicle if possible, onto the scale and the weights are noted. If each axle is weighed then the total weight (or gross weight) of the truck is then determined by adding the total of the axle weights. The weight of the vehicle's cargo can then be determined by subtracting the vehicle's empty weight (or tare weight) which is normally known beforehand, from the gross weight.
Although platform scales are quite accurate they are also relatively expensive to install. As a result, many shippers do not have them available on their premises, requiring operators to load the vehicles based on their estimates of the cargo weight. Then the operator must drive the vehicle to a scale installation at a remote location in order to weight it. If the vehicle is too heavy he must return to the shipper and off-load cargo. If the vehicle is underweight he must choose between returning for additional load or driving the vehicle with unused capacity. Obviously having to rely on guesswork in loading the vehicle can result in lost time and lost profit and is therefore undesirable.
Because of the disadvantages associated with fixed platform scales many operators now use portable scales in loading operations. Typically the scales are used in pairs with one scale being placed under each set of opposing wheels on an axle to obtain axle weight. After the vehicle has been loaded with the desired weight of cargo by estimate, it is then driven over the portable scales one axle at a time much in the same manner as done with the fixed platform scales. While these scales function reasonably well they have a number of disadvantages. First, unless the scales belong to the shipper the operator must carry them about in the vehicle which consumes additional weight and space. Secondly, they require the same time consuming loading process as do fixed platform scales where the vehicle is first loaded by estimate and then the load is adjusted to reach the desired actual weight. Further, some portable scales require that the vehicle tires be carefully positioned on the platform in order to obtain accurate weight readings which increases the time involved in the weighing process.
Because of the disadvantages associated with both fixed and portable platform scales a number of "onboard" weighing systems have been developed for trucks. Typically the design of such systems is based upon the theory that an increase of the weight of a vehicle will result in a proportional increase in the deflection or strain in certain structural members of the vehicle. In systems which correlate strain with load, strain changes in components of interest are normally measured using strain gages mounted on the surface of the components as is well known in the art. These systems were eagerly received by the industry because they offered the promise of providing instantaneous load information to an operator and appeared to eliminate most of the undesirable characteristics associated with platform scales. Unfortunately, most onboard systems found in the prior art have failed to live up that promise, not because of any fault in their underlying theory but because of deficiencies associated with implementation of the theory. For example, some systems relied on bending strain measurements from the vehicle's axles but these measurements proved to be an unreliable indicator of vehicle load. Such inaccuracies resulted from the fact that while axle bending strain was dependant upon load it was also dependent to some degree on the interface between the tires and the roadway.
Other systems relied upon strain measurements on components positioned between the vehicle's suspension and frame. While such systems provided more reliable load information they required expensive modifications to the vehicles. Some of these systems also experienced unacceptable rates of structural failure and required an undesirable increase in vehicle height. The displacement measuring systems found in prior art have also exhibited a number of significant disadvantages. For example, although component displacement may be dependent upon vehicle load it may also be dependent upon friction between components and wear of those components. Moreover, such systems often require expensive modifications in order to retrofit them to existing vehicles.
Accordingly, it is an object of this invention to provide for an onboard weighing system for trucks which is reliable and accurate.
It is a further object of this invention to provide for such an onboard weighing system which can be retrofitted to existing vehicles with minimal modifications and at a reasonable expense.
It is a further object of this invention to provide for such an onboard weighing system which is light in weight and compact so as not to significantly reduce the cargo volume or cargo carrying capacity of vehicles on which it is installed.